My wife is a primary school teacher, and related this tale after
another class returned from a trip to a working farm: 

My wife asked little David if he had enjoyed the trip. 
"Yes it was great - we saw sheep, horses, goats, and f***ers." 
Wife: "er, fine, fine. I know what the sheep and the rest are,
but what is a f***er?" 
David: "Oh, they're the animals that give us milk" 
Wife: "but who said they were called, er, f***ers?" 
David: "that was our teacher. Well actually she called them
"effers," but we all knew what she meant."